guys, this is simple. If the girl you like seems a bit sad then ask if she's ok. i mean, even if you two aren't official it shows that you care about her.Honestly! see how she reacts. If she tells you that she isn't ok then go on to make her laugh. If she tells you that she's fine.(they usually do.) you can believe her...OR you can tell her that she doesn't look fine so she can then open up to you and then she knows that she can talk to you and then, well, progress from that!